This invention relates to a time division telephone switching system.
A time division telephone switching system is generally constructed to exchange multiplexed signals which are sent over a plurality of incoming highways on a time division basis with time slots or channels of a plurality of outgoing highways. Such time division telephone exchange system is described, for example, in "The Bell System Technical Journal", Vol. 56, No. 7, 1977. The time division telephone switching system described in this journal has a construction as shown in FIG. 1 of the accompanying drawing. Briefly stated it comprises a trunk circuit 1 (hereinafter called TRK) which receives and transmits voice signals through voice frequency lines 2 (hereinafter called VL) and transmits line signals or channel associated signals, a CODEC 3 which converts analogue signals supplied from the trunk circuit 1 into digital time division multiplexed signals and a synchronizing circuit 4 (hereinafter called SYNC) which receives and transmits time division multiplexed signals over a digital line 5 and separates portions of the signals which contain the register signal and the line signal and transmits the separated portions. SYNC 4 synchronizes the frequency and phase of the time division multiplexed signal sent over the digital line 5 with a clock pulse of the time division telephone exchange and multiplexes and demultiplexes so as to coordinate the time division multiplexing of the time division telephone exchange with the time division multiplexed signal sent over the digital line 5. This time division telephone switching system further comprises a time division switching networks (TDNW) 6a and 6b for active mode and stand-by mode which are connected to CODEC 3 and SYNC 4 through changeover switches 7a and 7b respectively, and speech path controler (hereinafter called SPC) 8a and 8b for active and stand-by modes, which control the trunk circuit 1, SYNC 4 and the time division switching networks 6a and 6b. SPC 8a and 8b are connected to TRK 1, SYNC 4 and TDNW 6a and 6b through changeover switches 9, 10 and 11. Central processing units (CC) 12a and 12b of the active and stand-by modes are connected to SPC 8a and 8b through a changeover switch 13 for controlling all circuit elements through SPC 8a and 8b.
The time division telephone switching system described above receives a line signal sent over the voice frequency line 2 to TRK 1 which supplies the discriminating signal of a register signal and a channel associated signal to the active mode SPC 8a through the changeover switch 9. The voice frequency signal received by TRK 1 is supplied to CODEC 3 where it is converted into a time division multiplexed signal (including coincidence of the frequency and phase) which accords with predetermined conditions of the time division telephone exchange in the CODEC 3 and then applied to the active mode TDNW 6a. Under the control of CC 12a, SPC 8a controls TDNW 6a in accordance with a dial register signal and a line signal for transferring time division multiplexed voice signals allocated to respective time slots to time slots designated by the dial register signal.
The time slot containing the register signal and the line signal of the time division multiplexed signal is supplied via the digital line 5 and is separated by SYNC 4 and the separated time slot signal is applied to SPC 8a. SPC 8a identifies the register signal and the line signal by analysis of the signal supplied from SYNC 4 to control the time division switching network 6a under the control of CC 12a thus transferring the time division multiplexed voice frequency signal allocated to respective time slots to time slots designated by the register signal.
Where a fault occurs in any one of the time division switching unit 6a, the speech path control equipment 8a or the central processing unit 12a, a fault supervisory circuit, not shown, actuates corresponding one of the changeover switches 7a, 7b, 9, 10, 11 and 13 for changing over the faulty and places the stand-by unit in the active mode to a stand-by mode unit.
The switching operation described above is also performed between the voice frequency line 2 and the digital line 5.
With the time division telephone exchange described above, since the line signal supplied through the voice frequency line 2 is transmitted and received by TRK 1, and since the line signal supplied through the digital line 5 is transmitted and received by SYNC 4, the transmission and reception of these two signals and various control signals are processed differently so that the control procedure is extremely complicated and it is necesary to install a great number of signal lines between the many units and circuits. Accordingly, when it is desired to expand or contract the telephone network of such a time division switching system it is necessary not only to rearrange a large number of control lines but also to change various control procedures.